As methods for forming a circuit pattern on various substrates, a method which uses etching method with photolithography and a screen printing method have been conventionally applied as disclosed in Patent Documents 1 and 2. In these conventional methods, a method for subjecting a copper foil of a copper clad laminate to etching processing to form a circuit pattern, and a method for directly forming wiring and an electrode pattern on the surface of a substrate by screen printing using a conductive paste which is made to be paste by kneading metal powder with a solvent and a resin have been widely applied.
A method to directly form a circuit on the surface of the substrate by using a technique such as a screen printing method after processing metal powder into a paste (hereinafter, merely referred to as “conductive paste”) or an ink (hereinafter, merely referred to as “conductive ink”) have been widely spread. Because the method has fewer process steps than that of the etching method for subjecting the copper foil of the copper clad laminate to etching processing to form the circuit, and can reduce the production cost remarkably.
However, the formation of a finer circuit according to the demand of the miniaturization and weight reduction of electronic equipment or the like have been required for recent electric circuits. The largest problem in directly forming the circuit on the substrate using the conductive paste was in that it was difficult to form the fine circuit using the screen printing or the like. As disclosed in Patent Document 3, in recent years, the circuit formation by applying an inkjet technique that has been applied to a printer as a technique has been tried for forming the fine circuit using the conductive ink.
In recent years, a conductive circuit pattern in a flat display represented by a mobile information device and TV sets has been densified year after year. Examinations have been performed for wirings not only a region having a wiring width of 40 micron meter or less but also for a circuit pattern forming technique on a flexible resin substrate with low-temperature firing. The line width formed in conventionally used screen printing with no open circuit and excellent wiring shape is about 100 micron meter. However, it is difficult to substantially form the wiring finer than a line width of 40 micron meter or less. As a technique for forming a circuit pattern on various substrates with low-temperature firing, as shown in Patent Document 4, a silver ink containing silver nano particles has been examined.
On the other hand, a conductive ink prepared by mixing metal powder with a large quantity of organic solvents and resins has been proposed as an ultra fine circuit pattern formation material using a dispenser coating method and an inkjet printing method shown in Patent Document 3, adhesion strength to the various substrates depends on organic resins. So, minute cracks are easily generated by gas generation in the decomposition of organic resin contents in a reduction firing process using hydrogen and the nitrogen which are generally used for forming wiring and electrode having low resistance. Such a minute crack causes the low bulk density of the wiring or electrode, and as a result, it is difficult to form a circuit having low resistance.
On the other hand, Patent Document 5 discloses a conductive paste consist of an aqueous nickel slurry containing water, nickel fine powder on which an insoluble inorganic oxide is adhered to the particle surface of each fine nickel powder, a polyacrylic acid, an ester or a salt thereof, and organic group-substituted ammonium hydroxide, and a binder as a composition of a conductive ink. In the aqueous nickel slurry, nickel fine powder in a high concentration is dispersed stably without causing the reaggregation of the nickel fine powder. However, when the ultra fine circuit pattern is formed using the inkjet printing technique, the aqueous nickel slurry does not have a surface tension suitable for printing. Thereby, when the circuit is formed with continuous printing, the clogging of an ink is easily generated in a nozzle. Also, since a phenomenon in which the ink does not point at the target print position takes place, it was substantially difficult to form the circuit by continuous printing industrially. Since the ink does not contain a binder applying adhesion strength to the substrate, the adhesion strength to the substrate is substantially zero even if printing can be performed on the substrate by the arrangements of the printing process. Thereby, it was difficult to substantially form the circuit except for an application in which metal powder is sintered in high-temperature firing represented by the internal electrode production of a multilayered ceramic capacitor.    [Patent Document 1] Japanese Patent Laid-Open No. 9-246688    [Patent Document 2] Japanese Patent Laid-Open No. 8-18190    [Patent Document 3] Japanese Patent Laid-Open No. 2002-324966    [Patent Document 4] Japanese Patent Laid-Open No. 2002-334618    [Patent Document 5] Japanese Patent Laid-Open No. 2002-317201